A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Most lithographic apparatus are designed to be used with a radiation source that is as near monochromatic as possible. However, no radiation source can be perfectly monochromatic—the emitted radiation will have a certain spectral width which will affect the quality of the projected image since the projection and other optical systems will be chromatic. The spectral width of the source will also vary over time in ways that are not entirely predictable. For example, the spectral width of the output of a laser will vary due, for example, to thermal effects, to acoustic and resonant effects which depend on pulse repetition rate and due to aging of optical and electronic components. It is desirable therefore to monitor the spectral width of the source output so that appropriate control action can be taken. Such control is facilitated if the source spectrum can be represented by a single value. Known parameters of the source spectrum that are used for control in lithographic apparatus are the FWHM value and the E95 value. The FWHM (Full Width Half Maximum) value is the distance between the two points in the spectrum that have half the intensity of the peak. The E95 value is the width of the spectral peak that contains 95% of the energy of the peak and can be obtained by integrating the source spectrum and taking the distance between the points at which the integrated intensity is 2.5% and 97.5% of the total. FWHM and E95 are shown in FIG. 2 of the accompanying drawings, which shows intensity and integrated intensity vs. wavelength for an example of a spectral peak of a laser source. However, neither FWHM nor E95 is ideal as a parameter in all circumstances.